1. Field of Invention
This invention relates in general to air conditioning systems, and in particular to a thermostatic expansion valve.
2. Background of Related Art
A thermostatic expansion valve controls the flow of refrigerant through a closed loop refrigerant system. The thermostatic expansion valve senses the temperature and pressure of the refrigerant at the outlet of an evaporator and adjusts the opening and closing of a valve element within the thermostatic expansion valve to control the amount of refrigerant to the evaporator, and thus the superheat at the outlet of the evaporator.
The closed loop refrigeration system includes fluid conduits, a condenser, an evaporator, a compressor, and a thermostatic expansion valve. The thermostatic expansion valve includes a liquid line port (commonly known as Port A), an evaporator inlet port (commonly known as Port B), an evaporator outlet port (commonly known as Port C) and a suction line port (commonly known as Port D). The compressor compresses fluid refrigerant fluid within the closed loop system. The refrigerant then flows through the condenser. The condenser cools the refrigerant. The thermostatic expansion valve senses the temperature and pressure of the refrigerant exiting the evaporator and actuates a valve member within the thermostatic expansion valve for controlling the amount of refrigerant flowing from the condenser to the evaporator and thus achieving a desired superheat at the evaporator outlet. The refrigerant flows through the valve and into the evaporator where blown air is passed through the evaporator. The refrigerant absorbs heat from the air as it flow through the evaporator. The cooled air is used to cool the interior of a vehicle or a room.
A diaphragm within the thermostatic expansion valve separates two chambers (i.e., a charge chamber and a pressure chamber). The pressure differential on two sides of the diaphragm controls the opening and closing of the valve. When the pressure in the charge chamber is greater than the pressure in the pressure chamber, there is a net force on the diaphragm from the charge chamber to the pressure chamber, displacing fluid in the pressure chamber. In prior art designs, the pressure chamber is either in substantial fluid communication with a sensor chamber through a relatively wide open flow passage, or a structural extension of a sensor chamber that is situated between the evaporator outlet port and the suction line port. Therefore, in prior art designs, the pressure chamber pressure substantially follows the suction pressure at the sensor chamber.
During an initial period following a compressor startup, charge chamber temperature does not rapidly follow the evaporator outlet temperature, and as a result, the charge chamber pressure is relatively steady (i.e., drops slowly). On the other hand, the pressure chamber pressure drops rapidly with the suction pressure at a compressor startup. Since it takes longer for the charge chamber temperature to substantially reach its steady state than for the pressure chamber to substantially reach its steady state at the compressor startup, the thermostatic expansion valve opens rapidly and substantially, which also happens before the liquid line refrigerant is substantially sub-cooled. The diaphragm pushes a rapid rising valve open.